In the background art, as a paper double feed sensing device in a paper conveyance apparatus for conveying paper, there has been proposed a device in which the number of sheets of paper stacked in a paper stacking portion and then sent out therefrom is counted, the thickness of one sheet is obtained from the counted number of sheets and the change of height of stacked sheets of the paper, and a threshold value of the thickness is obtained. When the thickness of the paper measured on a paper conveyance path is thicker than the threshold value, the device regards the paper as double-fed.
(JP-A-06-040604)
There has been proposed another device in which the thickness of paper stacked in a paper stacking portion is measured when the paper is sent out, and a value of the average thickness is obtained and stored. When the thickness of paper measured thereafter is thicker than the stored value of the average thickness, the other device regards the paper as double-fed.
(JP-A-11-116101)
In the paper double feed sensing device disclosed in JP-A-06-040604, the number of sheets of paper stacked in a paper stacking portion and then sent out therefrom is counted. The thickness of one sheet of the paper is obtained from the counted number of sheets and a change of the height of the stacked paper, and a threshold value of the thickness is determined. Based on the threshold value, double feed is determined. It is therefore impossible to determine whether the paper sent out from the paper stacking portion for the first time has been double-fed or not.
In addition, the thickness of one sheet of the paper is obtained from the change of height of the stacked paper. It is therefore difficult to measure a change of the thickness corresponding to a few sheets accurately from the height of several hundreds of stacked sheets of the paper. It is therefore impossible to judge double feed correctly.
On the other hand, in the background art disclosed in JP-A-11-116101, the thickness of one sheet of paper stacked in a paper stacking portion is measured when the paper is sent out. An average value of the thickness is obtained and stored. When the paper thickness measured thereafter is thicker than the stored value, it is determined that the paper has been double-fed. Also in this case, it is difficult to determine whether the paper sent out for the first time has been double-fed or not.
Further, these paper double feed sensing devices cannot judge double feed correctly when sheets of paper different in thickness are mixed in the paper stacking portion.